Many types of transport vehicles (including vehicles for land, sea, air, or space travel) are being adapted for new and innovative applications in the field of commercial and military transportation systems. For example, station-keeping lighter-than-air airships (LTA) have been proposed to replace cell towers for wireless telephone and data systems. Military applications include use of high-altitude airship stations having electronic equipment for performing surveillance, intelligence gathering, radar, navigation, and similar other functions. In many such applications, long-duration station-keeping is essential. Thus, the airship must be equipped to provide power in a sustainable and reliable manner to operate payloads and power on-board systems while simultaneously making headway into the wind in order to hold a fixed geostationary position for desired time duration.
Solar cells capable of converting solar energy to electrical current are being used for powering some of the transport vehicles. In a traditional solar cell, energy from visible light that is incident on a photovoltaic cell is harvested by absorbing photons to generate charge carriers, e.g., holes and electrons, and then separating the charge carriers to drive a direct current (DC) to the electrodes of the solar cell. In a working solar cell, the charge carriers such as electrons must travel out of the solar cell and into an external electrical circuit, e.g., a load, to establish a current flow. Some of the electrons generated by the photons may be recaptured by holes before reaching the electrodes, thereby limiting the performance of the solar cell. Thus, providing a sustainable mechanism for the collection and physical transport of charge carriers is essential for the solar cell to operate reliably and efficiently.
Visible light corresponds to a wavelength of about 400 nanometers (violet color) to about 700 nanometers (red color). Many traditional solar cells are capable of absorbing energy from the visible light but are not capable of absorbing energy from other sources such as heat radiation. Heat radiation and blackbody radiation from the earth is continuous, day and night, with most of the energy in the wave length ranging from approximately 8 microns to approximately 40 microns, which may be up to 100× greater than the wavelengths of the visible light. Photons having less energy than a band gap (which defines the energy required to promote an electron from the valence band to the conduction band) for the material used in the solar cell pass through without being absorbed, while photons having energy higher than the band gap are absorbed, but their excess energy is wasted, e.g., typically dissipated as heat or vibrations.
Another challenge facing many traditional solar cells used in transport vehicles is low operating voltages. That is, many traditional solar cells have a low operating voltage, e.g., 2 volts or less, the voltage being limited by the energy of the incident photon. Voltage drops of 0.4 volts or higher within the solar cells may further limit the traditional solar cell to harvest energies of 0.4 eV or higher (obtainable from mid-infrared to visible light waves). Thus, the traditional DC current based solar cells may be able to harvest energy from heat or energy sources above approximately 600 degrees Celsius. Power-generating photovoltaic cells are not currently available that can deliver sustainable electrical power at a desirable voltage from energy sources operating at or below approximately 600 degrees Celsius. If such cells could be made, they could potentially supply the transports such as the high-altitude airship from the earthshine alone, continuously, day and night, without the need for solar cells (harvesting only visible light) or electrical energy storage.